Self-aligned nanotube field effect transistor and method of fabricating same

ABSTRACT

A self-aligned carbon-nanotube field effect transistor semiconductor device comprises a carbon-nanotube deposited on a substrate, a source and a drain formed at a first end and a second end of the carbon-nanotube, respectively, and a gate formed substantially over a portion of the carbon-nanotube, separated from the carbon-nanotube by a dielectric film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to field effect transistors, and moreparticularly, to carbon-nanotube field effect transistors.

2. Discussion of the Related Art

In the field of molecular nanoelectronics, few materials show as muchpromise as nanotubes, and in particular carbon nanotubes, which comprisehollow cylinders of graphite, angstroms in diameter. Nanotubes can beimplemented in electronic devices such as diodes and transistors,depending on the nanotube's electrical characteristics. Nanotubes areunique for their size, shape, and physical properties. Structurally acarbon-nanotube resembles a hexagonal lattice of carbon rolled into acylinder.

Besides exhibiting intriguing quantum behaviors at low temperature,carbon nanotubes exhibit at least two important characteristics: ananotube can be either metallic or semiconductor depending on itschirality (i.e., conformational geometry). Metallic nanotubes can carryextremely large current densities with constant resistivity.Semiconducting nanotubes can be electrically switched on and off asfield-effect transistors (FETs). The two types may be covalently joined(sharing electrons). These characteristics point to nanotubes asexcellent materials for making nanometer-sized semiconductor circuits.

In addition, carbon nanotubes are one-dimensional electrical conductors,meaning that only one-dimensional quantum mechanical mode carries thecurrent. This can be a significant advantage with respect to the deviceperformance of a carbon-nanotube based transistor since scattering inthe material is significantly suppressed. Less scattering means a betterperformance of the device.

For a three terminal device, such as an FET, a gate (the third terminal)needs to be isolated from the electrically active channel region as wellas a source and a drain. For this purpose a dielectric material, e.g.,silicon dioxide can be used. To improve device characteristics insilicon devices, the thickness of this layer can be reduced. Thisreduction increases the gate capacitance and improves thegate-to-channel coupling. For standard silicon field-effect devices thegate capacitance scales inversely proportional to the dielectric filmthickness. For currently manufactured high-performance processors, theSiO₂ thickness is less than 4 nm. Significantly, further reduction canbe difficult to achieve since gate leakage through the dielectric filmincreases exponentially for an oxide thickness below 4 nm.

However, the gate capacitance for a carbon-nanotube transistor does notscale inversely proportional with the dielectric film thickness.Instead, carbon-nanotubes follow a logarithmic scaling law. Incomparison with a standard silicon field-effect transistor, the gatecapacitance for a carbon-nanotube transistor can be larger because ofthe cylindrical geometry of these objects.

No known system or method has implemented a nanotube to achieveperformance and smaller size in an FET. Therefore, a need exists for asystem and method of preparing nanotube based FETs.

SUMMARY OF THE INVENTION

According to an embodiment of the present invention, a self-alignedcarbon-nanotube field effect transistor semiconductor device isprovided. The device comprises a carbon-nanotube deposited on asubstrate, a source and a drain formed at a first end and a second endof the carbon-nanotube, respectively, and a gate formed substantiallyover a portion of the carbon-nanotube, separated from thecarbon-nanotube by a dielectric film.

The substrate comprises a thermal oxide deposited over a siliconsubstrate. The thermal oxide is about 150 nanometers thick.

The gate is further separated from the carbon-nanotube by an oxidelayer. A portion of the gate is separated from the source and the drainby a nitride spacer.

The device further comprises a passivation dielectric layer over thedevice.

The device comprises an alignment mark in the substrate to which thesource and the drain are aligned.

The gate wraps around the dielectric film and the carbon-nanotube tocontact a back side of the carbon-nanotube.

According to an embodiment of the present invention, a carbon-nanotubefield effect transistor semiconductor device is provided. The devicecomprises a vertical carbon-nanotube wrapped in a dielectric material, asource and a drain formed on a first side and a second side of thecarbon-nanotube, respectively, a bilayer nitride complex through which aband strap of each of the source and the drain is formed connecting thecarbon-nanotube wrapped in the dielectric material to the source and thedrain, and a gate formed substantially over a portion of thecarbon-nanotube.

The device comprises a metal catalyst at a base of the carbon-nanotube.

According to one embodiment of the present invention, a method isprovided for forming a self-aligned carbon-nanotube field effecttransistor semiconductor device. The method comprises depositing ananotube on a thermal oxide substrate, wherein the substrate includes analignment mark, forming a metal contact at each end of the nanotube,wherein a first metal contact is a source and a second metal contact isa drain, and depositing an amorphous silicon layer over the device. Themethod further comprises forming nitride spacers on opposing sides ofeach metal contact, depositing a high k dielectric film over the device,oxidizing the amorphous silicon, and forming a gate substantiallybetween the source and the drain, and over the nanotube.

The method comprises depositing a passivation dielectric over thedevice.

The nanotube is a single-walled nanotube. The metal contacts are formedusing a photoresist.

According to an embodiment of the present invention, a method isprovided for forming a self-aligned carbon-nanotube field effecttransistor semiconductor device. The method comprises depositing ananotube on a thermal oxide substrate, wherein the substrate includes analignment, mark, forming a metal contact by reactive ion etch at eachend of the nanotube, wherein a first metal contact is a source and asecond metal contact is a drain, and forming nitride spacers on opposingsides of each metal contact. The method further comprises depositing ahigh k dielectric film over the device, and forming a gate substantiallybetween the source and the drain and over the nanotube.

The method comprises depositing a passivation dielectric over thedevice.

According to an embodiment of the present invention, a method isprovided for forming a self-aligned carbon-nanotube field effecttransistor semiconductor device. The method comprises depositing ananotube on a thermal oxide substrate, wherein the substrate includes analignment mark, and forming an amorphous silicon pillar over each end ofthe nanotube. The method further comprises isolating the amorphoussilicon pillars with a layer of oxide, forming a gate dielectric layerbetween amorphous silicon pillars, and forming a gate substantiallybetween the amorphous silicon pillars and over the nanotube. The methodcomprises forming a nitride layer over the gate, forming oxide spacerson each side of the gate, replacing the amorphous silicon with metalcontacts, wherein a first metal contact is a source and a second metalcontact is a drain, and depositing a passivation dielectric over thedevice.

According to another embodiment of the present invention, a method isprovided for forming a self-aligned carbon-nanotube field effecttransistor semiconductor device. The method comprises depositing a metalcatalyst on a thermal oxide substrate, depositing a low temperatureoxide layer over the device, etching a trench through the oxide, themetal catalyst and into a thermal oxide underlying the metal catalyst,and etching the low temperature oxide layer to form oxide islands. Themethod further comprises stripping exposed metal catalyst, growing ananotube between metal catalyst beneath the oxide islands, and wrappingthe nanotube in a gate dielectric. The method comprises forming nitridespacers on the opposing surfaces of the oxide islands, forming a gatesubstantially between the oxide islands by chemical vapor deposition andover the nanotube, and depositing a passivation dielectric over thedevice.

According to an embodiment of the present invention, a method isprovided for forming a self-aligned carbon-nanotube field effecttransistor semiconductor device. The method comprises growing a nanotubevertically from a metal catalyst forming on a surface of thesemiconductor device, forming a nitride block structure, and wrappingthe nanotube in a gate dielectric. The method comprises depositing agate metal separated from the metal catalyst by the dielectric layer,depositing a nitride layer, and forming gate metal pillars capped withthe nitride layer. The method forms nitride spacers around the pillars,deposits a drain metal substantially between the pillars separated fromthe gate metal by the dielectric layer, and deposits a passivationdielectric over the device.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will be described belowin more detail, with reference to the accompanying drawings:

FIGS. 1 a-i illustrate a source/drain first carbon-nanotube field effecttransistor according to an embodiment of the present invention;

FIGS. 2 a-b illustrate another source/drain first carbon-nanotube fieldeffect transistor according to an embodiment of the present invention;

FIGS. 3 a-g illustrate a gate first carbon-nanotube field effecttransistor according to an embodiment of the present invention;

FIGS. 4 a-d illustrate a carbon-nanotube field effect transistorcomprising a nanotube grown in place according to an embodiment of thepresent invention;

FIGS. 5 a-n illustrate a carbon-nanotube field effect transistorcomprising a nanotube grown in place vertically according to anembodiment of the present invention; and

FIGS. 6 a-b illustrate directed assembly of nanotubes according to anembodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

According to an embodiment of the present invention, a gate, a sourceand a drain of a field effect transistor (FET) are self-aligned, therebyreducing overlap capacitances.

According to an embodiment of the present invention, a carbon-nanotubeFET can be fabricated using a pattern transfer by lift-off etch, whereinthe source and the drain are formed before the gate. Referring to FIGS.1 a-i, an alignment mark 101 is formed in a thermal oxide 102 andsilicon substrate 103. The alignment mark 101 is a high precisionfeature that can be used as a reference when positioning patterns. Thethermal oxide 102 is deposited over the silicon 103. The silicon can be,for example, P+ doped (0.01Ω-cm, about 3×10¹⁸cm⁻³). A nanotube 104 canbe deposited on the thermal oxide 102 and a photoresist 105 can bepositioned by photolithography. The nanotube can be deposited in theform of a slurry, where nanotube deposition is random. The nanotube canbe deposited by directed assembly, as described below. The photoresistexposes the ends of the nanotube. Metal contacts 106-107 are formed inthe trenches that expose the nanotube ends. The metal can be, forexample, Cobalt (Co), Nickel (Ni), Tungsten (W), or Titanium (Ti). Themetal can be deposited over the device, filling the trenches exposingthe ends of the nanotube 104. The photoresist 105 can be stripped. Themetal deposited in the trenches form source/drain contacts 106-107. Anamorphous Silicon (a-Si) 108 can be deposited over the device. A Nitridelayer 109 can be deposited over the a-Si layer. The Nitride can beetched to form spacers, e.g., 110 on the sides of the metal contacts106-107. The amorphous silicon 108 can be selectively removed or wetchemically oxidized. A gate dielectric film 111 can be deposited overthe device. Here as in the following methods, the dielectric can besilicon dioxide as well as-any other high-k dielectric material, forexample, HfO₂. A gate 112 can be formed substantially between the metalcontacts 106-107 forming the source and drain, for example by CVD andetching. A passivation dielectric layer 113 is deposited over thedevice. The source, drain and gate 112 are self-aligned to the alignmentmark 101.

Alternatively, the source/drain can be formed before the gate with areactive ion etch (RIE). Referring to FIGS. 2 a-b, a method forms thesource/drain, 106-107, first using RIE to define the source/drain metal.The RIE needs to be isolated from the carbon-nanotube 104. A nitridelayer 201 can be deposited over the device and etched from the areassurrounding the metal contact. Nitride spacers, e.g., 202, can be formedon the sides of the metal contacts. A gate dielectric 203 is depositedover the device. The gate metal 204 is formed substantially between thesource and the drain, 106-107. A passivation dielectric 205 can bedeposited over the device. The thermal oxide can be approximately 150 nmthick.

According to another embodiment of the present invention, the gate canbe formed before the source/drain. Amorphous silicon 301 can bedeposited over the ends of the nanotube 104. The a-Si can be coveredwith an oxide layer 302. A gate dielectric 303 can be deposited betweenthe a-Si, e.g., 301. A gate 304 can be formed substantially between thea-Si pillars, e.g., 301. A nitride layer 305 can be formed over the gatemetal 304. Oxide spacers, e.g., 306 can be formed on the ends of thegate metal 304. The exposed corners of the a-Si/oxide can be stripped,exposing the a-Si. The remaining a-Si surrounding the gate metal can beremoved by RIE. Metal contacts 307-308 can be deposited in the areapreviously occupied by the a-Si. The metal contacts 307-308 areconnected to the nanotube 104 that runs beneath the gate dielectric 303and gate metal 304. The metal contacts 307-308 form the source and thedrain of the device. The metal contacts 307-308 can be aligned to thealignment mark 101 deposited in the thermal oxide 102 and silicon 103substrate. A passivation dielectric 309 can be deposited over thedevice.

According to an embodiment of the present invention, a carbon-nanotubeFET can be grown in place. The source/drain can be formed before thegate. An amorphous silicon layer 401 is deposited over the thermal oxidelayer 102. A low temperature oxide (LTO) layer 402 can be deposited overthe metal catalyst. A trench can be etched from the oxide 402, amorphoussilicon 401 and thermal oxide 102. The amorphous silicon 401 can bepartially under cut from below the oxide 402. A metal catalyst 401B, forexample, Fe, Co, Ni or Fe/Mo can be self-assembled on the edges of theundercut amorphous silicon film 401. The carbon-nanotube 403 can begrown between the remaining portions of the metal catalyst 401B, whereina portion of the nanotube is suspended over the thermal oxide 102. Agate dielectric film 404 can be deposited by chemical vapor deposition(CVD), wrapping the nanotube 403. Thus, the nanotube 403 can becompletely covered with the gate dielectric, e.g., SiO₂. Spacers, e.g.,405, can be formed on the sides of the oxide, e.g., 402. A gate 406 canbe formed substantially between the oxide, e.g., 402. If the etch in thethermal oxide 102 is sufficiently deep, the gate metal 406 can surroundthe whole nanotube 403 and the dielectric film 404 stack. For thispurpose the gate metal can be deposited by means of chemical vapordeposition to cover the back side of the nanotube/dielectric film stack.The wrapped around configuration offers a good gate-to-nanotubecoupling. A passivation dielectric 406 can be deposited over the surfaceof the device.

According to another embodiment of the present invention, acarbon-nanotube can be grown in place vertically. The nanotube can begrown vertically from, for example, a metal source at the base or ametal particle catalyst. Referring to FIG. 5 a-n, a metal catalyst 501can be formed on the silicon substrate 502. A first layer of Nitride 503can be deposited over the device. An oxide-layer 504 can be depositedover the first layer of Nitride 503. A second layer of Nitride 505 canbe deposited over the oxide 504. A photoresist, e.g., 506, can be formedon the device by photolithography, wherein the metal catalyst 501 isexposed. A plurality of second metal catalysts, e.g., 507, are depositedover the device. The photoresist, e.g., 506, can stripped, such that thesecond catalyst, e.g., 507, formed on the first metal catalyst 501remain. From each second metal catalyst, e.g., 507, a nanotube, e.g.,508, can be grown vertically. Thus, two-dimensional andthree-dimensional arrays of nanotubes can be formed.

Vertical growth of the nanotubes occurs when the metal particle catalystis placed in a pore aligned vertically to the substrate. In this case,the space for the growth is confined and forces the growth of the tubeto follow the vertical direction. In principle, vertical pores such asin FIG. 5 b can be made using the resists and pattern transfer.

An amorphous Silicon layer 509 can be deposited over the device. Thedevice can be planarized down to the second Nitride layer 505. A portionof the Nitride-Oxide-Nitride layering, 503-505, can be removed from thedevice. A pillar surrounding the nanotubes, e.g., 508, and metalcatalyst, 501 and 507, remains. A sacrificial layer 510 can be formedover the Nitride layer 505 the nanotubes 508 and the a-Si 509. Thecontact layer can be, for example, titanium or tungsten. The oxide layer504 can be removed from between the layers of Nitride, 503 and 505. Thea-Si 509 can be etched simultaneously with the oxide layer 504 fromaround the nanotubes, e.g., 508. Alternatively, the a-So 509 can beremoved after the oxide layer 504 has been removed. A gate dielectric,e.g., 511, can be formed around the nanotubes, e.g., 508, over the metalcatalyst 501 and under the sacrificial layer 510. Alternatively, for atwo-dimensional array of nanotubes, the gate dielectric 511 can bedeposited between the nanotubes. The sacrificial layer 510 can beremoved, for example, by an etch. The gate metal 512 can be depositedover the surface of the device. A third Nitride layer 513 can bedeposited over the gate metal 512. Portions of the gate metal 512 andthe third nitride layer 513 can be removed. Pillars of gate metal andnitride spacers, e.g., 512 and 513, remain around the metalcatalyst-nanotube structure. Nitride spacers, e.g., 514, are formedaround each pillar. A drain 515 can be formed over the metalcatalyst-nanotube structure, forming a FET. The passivation dielectric516 can be deposited between FETS.

It should be noted that the exact mechanics of nanotube growth from ametal catalyst are not known. However, the process of growing asingle-walled nanotube from a metal catalyst, for example, Cobalt (CO)over alumina-supported Molybdenum (Mo) particles, can be implemented ina number of ways.

According to an embodiment of the present invention, nanotubes can beput in place by a method of directed assembly rather than deposited orgrown as described above. Directed assembly can be used for horizontaland vertical deposition of a nanotube using selective deposition drivenby a chemical or a physical process. The selective deposition caninclude forming an adhesion layer or chemical groups acting as receptorsto favor a desired deposition of tubes in a given position. FIGS. 6 aand 6 b show methods for horizontal and vertical directed assembly,respectively. A nanotube 601 can be prepared comprising predeterminedchemical groups 602, for example, a DNA strand or a thiol group, at eachend. The nanotube 601 can be brought into the proximity of a substrate603 comprising receptors 604, for example, where DNA is implemented, acomplementary DNA strand can be used. Where a thiol group is used, goldparticles or a contact shape comprising gold, can be designed to bondwith the chemical groups 602 of the nanotube 601. The nanotube 601 canthus be placed on the substrate 603 according to directed assembly.

For improved performance high-K dielectric films, those having a highdielectric constant, can be used as gate insulators. The capacitance ofa carbon-nanotube FET does not significantly change as a function of thethickness of the dielectric film, thus, it can be difficult to achievethe desired capacitance, even with thin gate dielectric films. Aluminumoxide Al₂O₃ (k=9) as well as Hafnium oxide (HfO₂) (k=20) are promisingcandidates in this context. CVD-aluminum can be oxidized to generate ahigh-K gate dielectric or CVD-Al₂O₃(HfO₂ can be deposited directly.Compared with SiO₂, these materials increase the gate capacitance by afactor of up to five, and can have a larger impact on the deviceperformance than reducing the dielectric film thickness. Since nanotubesare pFETs in an air environment and become nFETs in vacuum and inertgases like Argon (Ar) after annealing, the device can be annealed beforethe deposition of the dielectric film is added. This converts the tubesinto nFETs. Capping them in situ with the dielectric also prevents tubesfrom becoming pFETs again. For a complementary technology the dielectricfilm on FETS, which should be turned into pFETs, can be locallyremoved—also allowing the FETs to be doped. A CVD deposition at lowtemperature coats these devices again (without an extra annealing stepbefore).

Since all structures (pFETs and nFETs) are covered with oxide (or anyother suitable dielectric film) no short is generated when the gateelectrode is fabricated. CVD can be used for the deposition of the gate.Using chemical vapor deposition for fabrication schemes as described inFIGS. 4 and 5 can ensure that nanotubes that are already wrapped in adielectric film become completely surrounded by the metal gate. This canbe important for good gate-to-nanotube coupling. The gate metal can bepatterned and removed where desired. Source and drain electrodes can beopened for electrical access.

Having described preferred embodiments for carbon-nanotube FETs andmethods of making same, it is noted that modifications and variationscan be made by persons skilled in the art in light of the aboveteachings. It is therefore to be understood that changes may be made inthe particular embodiments of the invention disclosed which are withinthe scope and spirit of the invention as defined by the appended claims.Having thus described the invention with the details and particularityrequired by the patent laws, what is claimed and desired protected byLetters Patent is set forth in the appended claims.

1-7. (canceled)
 8. A self-aligned carbon-nanotube field effecttransistor semiconductor device comprising: a carbon-nanotube depositedon a substrate: a source formed at a first end of the carbon-nanotube: adrain formed at a second end of the carbon-nanotube: and a gate formedsubstantially over a portion of the carbon-nanotube, separated from thecarbon-nanotube by a dielectric film, wherein the gate wraps around thedielectric film and the carbon-nanotube to contact a back side of thecarbon-nanotube.
 9. A carbon-nanotube field effect transistorsemiconductor device comprising: a vertical carbon-nanotube wrapped in adielectric material; a source formed at a first side of thecarbon-nanotube; a drain formed at a second side of the carbon-nanotube;a bilayer nitride complex through which a band strap of each of thesource and the drain is formed connecting the carbon-nanotube wrapped inthe dielectric material to the source and the drain; and a gate formedsubstantially over a portion of the carbon-nanotube.
 10. Thecarbon-nanotube field effect transistor semiconductor device of the 9,further comprising a metal catalyst at a base of the carbon-nanotube.11. A method for forming a self-aligned carbon-nanotube field effecttransistor semiconductor device comprising the steps of: depositing ananotube on a thermal oxide substrate, wherein the substrate includes analignment mark; forming a metal contact at each end of the nanotube,wherein a first metal contact is a source and a second metal contact isa drain; depositing an amorphous silicon layer over the device; formingnitride spacers on opposing sides of each metal contact; depositing ahigh k dielectric film over the device; oxidizing the amorphous silicon;and forming a gate substantially between the source and the drain, andover the nanotube.
 12. The method of claim 11, further comprising thestep of depositing a passivation dielectric over the device.
 13. Themethod of claim 11, wherein the nanotube is a single-walled nanotube.14. The method of claim 11, wherein the metal contacts are formed usinga photoresist.
 15. A method for forming a self-aligned carbon-nanotubefield effect transistor semiconductor device comprising the steps of:depositing a nanotube on a thermal oxide substrate, wherein thesubstrate includes an alignment mark; forming a metal contact byreactive ion etch at each end of the nanotube, wherein a first metalcontact is a source and a second metal contact is a drain; formingnitride spacers on opposing sides of each metal contact; depositing ahigh k dielectric film over the device; and forming a gate substantiallybetween the source and the drain and over the nanotube.
 16. The methodof claim 15, further comprising the step of depositing a passivationdielectric over the device.
 17. A method for forming a self-alignedcarbon-nanotube field effect transistor semiconductor device comprisingthe steps of: depositing a nanotube on a thermal oxide substrate,wherein the substrate includes an alignment mark; forming an amorphoussilicon pillar over each end of the nanotube; isolating the amorphoussilicon pillars with a layer of oxide; forming a gate dielectric layerbetween amorphous silicon pillars; forming a gate substantially betweenthe amorphous silicon pillars and over the nanotube; forming a nitridelayer over the gate; forming oxide spacers on each side of the gate;replacing the amorphous silicon with metal contacts, wherein a firstmetal contact is a source and a second metal contact is a drain; anddepositing a passivation dielectric over the device.
 18. A method forforming a self-aligned carbon-nanotube field effect transistorsemiconductor device comprising the steps of: depositing a metalcatalyst on a thermal oxide substrate; depositing a low temperatureoxide layer over the device; etching a trench through the oxide, themetal catalyst and into a thermal oxide underlying the metal catalyst;etching the low temperature oxide layer to form oxide islands; strippingexposed metal catalyst; growing a nanotube between metal catalystbeneath the oxide islands; wrapping the nanotube in a gate dielectric;forming nitride spacers on the opposing surfaces of the oxide islands;forming a gate substantially between the oxide islands by chemical vapordeposition and over the nanotube; and depositing a passivationdielectric over the device.
 19. A method for forming a self-alignedcarbon-nanotube field effect transistor semiconductor device comprisingthe steps of: growing a nanotube vertically from a metal catalystforming on a surface of the semiconductor device; forming a nitrideblock structure; wrapping the nanotube in a gate dielectric; depositinga gate metal, separated from the metal catalyst by the gate dielectric;depositing a nitride layer; forming gate metal pillars capped with thenitride layer; forming nitride spacers around the pillars; depositing adrain metal substantially between the pillars, separated from the gatemetal by the dielectric layer; and depositing a passivation dielectricover the device.